Color liquid crystal display

ABSTRACT

In an ECB LCD, in which a liquid crystal layer enclosed between a pair of substrates is driven based on R, G, B signals so that transmittance of R, G, B light components at the liquid crystal layer is controlled for color display, the voltage levels of the liquid crystal driving signals for R, G, B light are set such that the optimum transmittance, i.e., the maximum transmittance, can be achieved with respect to the R, G, B light components. With this arrangement, wavelength dependency, if any, of the liquid crystal with respect to the light coming into the light crystal layer can be modified so that color display is achieved with superior color reproducibility.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a color liquid crystal display(LCD) which employs a voltage control birefringence method forcontrolling the tilt direction of, for example, vertically-alignmentliquid crystal by using an electric field.

[0003] 2. Description of the Prior Art

[0004] LCDs, in which liquid crystal is enclosed between a pair ofsubstrates and a voltage is applied to the enclosed liquid crystal fordesired display are advantageously small and thin, and the powerconsumption thereof can be easily reduced. Due to these advantages, LCDsare widely used as displays in various office automation equipment, suchas personal computers, and audio visual equipment, such as a projector,and portable or on-board devices.

[0005] In particular, a DAP (deformation of vertically aligned phase)LCD is proposed, which includes liquid crystal with negative dielectricconstant anisotropy, and controls initial alignment of the liquidcrystal molecules so as to be vertically-aligned by using a verticalalignment layer. Specifically, a DAP LCD employs one type ofelectrically controlled birefringence (ECB) methods, and controlstransmittance and displayed colors of the light coming into the liquidcrystal layer by utilizing a difference in a reflective index betweenthe longer and shorter axes of a liquid crystal molecules, i.e., abirefringence phenomenon. A pair of substrates are provided each with apolarization film attached on the outer surface thereof, such that theirpolarization directions are orthogonal to each other. When voltage isapplied to the liquid crystal layer, linearly polarized light which hasbeen introduced into the liquid crystal layer via the polarization filmon one side of the substrate is converted into elliptically orcircularly polarized light due to birefringence thereof of the liquidcrystal layer, and is partly ejected from the polarization film on theother side. Since the extent of birefringence of the liquid crystallayer, i.e., a phase difference (a retardation amount) between ordinaryand extraordinary ray components of the incoming linearly polarizedlight, is determined according to the voltage applied to the liquidcrystal layer, i.e., the intensity of an electric field generated in theliquid crystal, the amount of light ejected from the second polarizationfilm can be controlled for every pixel by controlling for every pixelthe voltage applied to the liquid crystal layer. This eventually makesit possible to display a desired color image when RGB color filters andRGB light sources are used.

DESCRIPTION OF THE RELATED ART

[0006] A DAP LCD can be fabricated without a rubbing step for giving theliquid crystal vertical alignment in a manufacturing process as a resultof improvement of a panel structure. The DAP method can therefore beemployed in an LCD which includes a thin film transistor (TFT) as aswitching element for driving each liquid crystal element and as adriver for driving the switch element.

[0007] However, an LCD having a structure which can make optimum use ofor even improve the characteristics of a low temperature poly silicon(p-Si) TFT and those of the DAP method is yet to be developed foroptimization.

[0008] For example, while being superior in having a wider viewing angleand originally-high transmittance of incoming light, the DAP method hasunfortunately dependency on the wavelength of incoming light since theamount of permeated light transmittance is determined depending on thebirefringence extent of the liquid crystal layer, such as Δn·d/λ(Δn:variation of refractive index of liquid crystal layer, d: thickness ofliquid crystal layer; λ wavelength of incoming light). It is possible toreduce the wavelength dependency through, for example, adjustment of thethickness d of the liquid crystal layer as the wavelength dependency isweaker for a larger Δn·d. However, since such adjustment may causeunfavorable effects in view of parallax, control of the wavelengthdependency through adjustment of the thickness d of the liquid crystallayer is subject to limitation. On the other hand, in view of a lowerpower consumption, a study is being made to develop a material which issuperior in response characteristics respect with a low voltage. Also, amaterial which has a smallerΔn than conventional liquid crystal materialmay be used. In the case of a reflection LCD, the valueΔn·d maynecessarily be small due to the characteristics of the device.Therefore, wavelength dependency of an LCD with respect to incominglight cannot be considered negligible as it may adversely affect thequality of color display, particularly, in view of colorreproducibility.

SUMMARY OF THE INVENTION

[0009] The present invention has been conceived to overcome the aboveproblems and aims to provide a device which enables displaying, inparticularly, color-displaying with high quality through modification ofwavelength dependency of an LCD with respect to incoming light.

[0010] According to a first aspect of the present invention, there isprovided a liquid crystal display having liquid crystal sandwiched by apair of substrates having electrodes for driving the liquid crystalbased on a liquid crystal control driving signal for R light, a liquidcrystal control driving signal for G light, and a liquid crystal controldriving signal for B light to control transmittance of R lightcomponents, G light components, and B light components for colordisplay, wherein a driving voltage for application to the liquid crystalis set independently for R display, G display, and B display.

[0011] According to a second aspect of the present invention, there isprovided an ECB liquid crystal display having liquid crystal sandwichedby a pair of substrates having electrodes for driving the liquid crystalbased on a liquid crystal control driving signal for R light, a liquidcrystal control driving signal for G light, and a liquid crystal controldriving signal for B light to control transmittance of R lightcomponents, G light components, and B light components for colordisplay, wherein a driving voltage for application to the liquid crystalis set independently for R display, G display, and B display.

[0012] Further, in the above liquid crystal display, an upper limitvalue of a range for the driving voltage is set independently for Rlight, G light, and B light.

[0013] When a driving voltage is independently controlled for R, G, Blight, wavelength dependency with respect to incoming light can bemodified in color display in which the white color is displayed throughcomposition of R, G, B light, so that adequate color reproducibility canbe easily realized regardless the difference of the liquid crystalmaterial in use or the thickness of the liquid crystal layer.

[0014] Further, in the above liquid crystal display, the liquid crystalcontrol driving signal for R light, the liquid crystal control drivingsignal for G light, and the liquid crystal control driving signal for Blight are separately subjected gamma correction based on transmittancecharacteristics of the R light components, the G light components, andthe b light components.

[0015] When gamma correction is carried out for the liquid crystalcontrol driving signals for R, G, B light according to the respectivecharacteristics of the R, G, B light, color reproducibility can beenhanced even with respect to intermediate graduation. This enablesliquid crystal color display with higher quality.

[0016] Still further, in the above liquid crystal display the pair ofsubstrates includes a first substrate, electrodes for driving the liquidcrystal formed on the first substrate include a plurality of pixelelectrodes arranged in a matrix thereon; and the plurality of pixelelectrodes are connected to corresponding p-Si thin film transistorseach using a p-Si layer formed at a low temperature for an active layer.

[0017] When a p-Si thin film transistor is used as a switching elementfor each liquid crystal pixel, it is possible to drive each pixel of theLCD by a low voltage to display a very fine image.

[0018] As described above, according to the present invention, liquidcrystal driving signals for R, G, B are adjusted according to therespective transmittance characteristics, so that respective colors canbe favorably reproduced for display even by an LCD with high wavelengthdependency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and the other objects, features, and advantages of thepresent invention, will become further apparent from the followingdescription of the preferred embodiment taken in conjunction with theaccompanying drawings wherein:

[0020]FIG. 1 is a conceptual plan view showing an example of a structureof an LCD panel according to a preferred embodiment;

[0021]FIG. 2 is a schematic cross sectional view of the LCD panel alongthe A-A line in FIG. 1;

[0022]FIG. 3 is a block diagram showing a global structure of the LCDaccording to the preferred embodiment;

[0023]FIG. 4 is a graph showing wavelength dependency of an impressedvoltage and transmittance with the LCD panel according to the preferredembodiment;

[0024]FIG. 5 is a schematic diagram showing a structure of an RGB driverprocessing circuit 70 shown in FIG. 3;

[0025]FIG. 6 is a diagram showing a structure of a limit levelgeneration circuit 84 shown in FIG. 5;

[0026]FIG. 7 is a diagram showing waveforms of signals in the circuitshown in FIG. 5; and

[0027]FIG. 8 is a diagram showing a structure of a projector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] In the following, a preferred embodiment of the present invention(hereinafter referred to as a preferred embodiment) will be describedwith reference to the accompanying drawings. In this embodiment, an ECBLCD or a DAP LCD is used, in which an ECB LCD controls alignment ofliquid crystal by using a electric field so that a transmittance amountis controlled by utilizing a birefringence phenomenon, and the DAP LCDis one type of the ECB LCDs. Specifically, when using a DAP LCD, R, G, Bliquid crystal driving signals are controlled based on the transmittancecharacteristics of corresponding R, G, B light components whereby R, G,B liquid crystal voltage levels (an ON display level) are controlled.

[0029] [LCD panel structure]

[0030] Referring to FIGS. 1 and 2, a structure of a DAP LCD panel to bedriven will be described. FIG. 1 shows one example of a plan structureof an LCD panel; FIG. 2 shows one example of a schematic cross sectionalstructure of the LCD panel along the line A-A shown in FIG. 1. An LCDaccording to the preferred embodiment comprises a TFT substrate (a firstsubstrate) 10 and an opposing substrate (a second substrate) 30. On thefirst substrate 10, a lower temperature poly silicon (p-Si) TFT isformed, above which a pixel electrode 26 is further formed. On thesecond substrate 30, which is provided opposing the first substrate 10having a liquid crystal layer 40 in-between, a common electrode 32 isformed which has an direction control window 34 thereon. Further,polarization films 44, 46 are attached to the outer sides of therespective substrates 10, 30 such that respective transmittancepolarized light directions are orthogonal to each other.

[0031] More specifically, on the TFT substrate 10 which is made of glassor other material, there are formed a gate electrode 12 and a gateelectrode line 12L which is integral to the gate electrode 12 in thisembodiment. The gate electrode 12 and the line 12L are formed bypatterning metal such as Cr, Ta, Mo, and so on. Further, covering thegate electrode 12 and the line 12L, a gate insulating film 14 is formedwhich is made of either one of SiNx and SiO₂ or layers of thesematerials stuck on top of each other. On the gate insulating film 14, ap-Si thin film 20 is formed to serve as an active layer of the TFT. Thep-Si thin film 20 is formed by annealing, at a low temperature, anamorphous silicon (a-Si) thin film through both or either laser and/orlamp annealing for poly-crystallization and patterning into an islandshape after the annealing.

[0032] On the p-Si thin film 20, an implantation stopper (dopingstopper) 23 is formed which is made of SiO₂ or other material. Thedoping stopper 23 is formed through self-alignment so as to a shapesubstantially identical to the gate electrode 12 when the TFT substrate10 is exposed to light from its rear side (the bottom side in FIG. 2)while using the gate electrode 12 as a mask. Further, when impurities,such as, phosphorous or arsenic, are doped into the p-Si thin film 20 tobe at a low concentration by using the injection stopper 23 as a mask, alow concentration source region 20LS and a low concentration drainregion 20LD are formed through self-alignment on the respective sides ofthe region directly below the doping stopper 23, of the p-Si thin film20. The low concentration source region 20LS and the low concentrationdrain region 20LD contain the doped impurities at a low concentration.The region directly below the doping stopper 23 of the p-Si thin film 20contains no impurities because the doping stopper 23 served as a mask atthe time of ion doping, so that the region constitutes an intrinsicregion serving as a channel region 20CH of the TFT. Further, a sourceregion 20S and a drain region 20D are formed on the outer sides of thelow concentration source region 20LS and the low concentration drainregion 20LD, respectively, from the doping stopper 23 when the identicalimpurities are further doped therein to be at a higher concentration.

[0033] On the doping stopper 23 and the p-Si thin film 20 where therespective regions (20CH, 20LS, 20LD, 20S, 20D) have been formed in theabove processes, an inter-layer insulating layer 22 made of SiNx orother material is formed covering these regions. Further, on theinter-layer insulating film 22, a source electrode 16, a drain electrode18, and a drain electrode 18L which is integral to the drain electrode18 are formed, made of Al, Mo, or other material. The source electrode16 and the drain electrode 18 are connected via contact holes openedthroughout the inter-layer insulating film 22 to the low concentrationsource region 20S and the low concentration drain region 20D,respectively.

[0034] A low temperature p-Si TFT of this embodiment comprises theaforementioned gate electrode 12, the gate insulating film 14, the p-Sithin film 20 (20CD, 20LS, 20LD, 20S, 20D), the source electrode 16, andthe drain electrode 18. The TFT further comprises an active layer madeof a p-Si thin film 20 having been formed in low temperature processing.Although the above TFT is of a bottom-gate type in which a gateelectrode 12 is positioned on the lower side of the elements, a TFT isnot limited thereto and may be of a top-gate type in which a gateelectrode is formed in a layer above the p-Si thin film.

[0035] On substantially the entire part of the TFT substrate 10, aflattening inter-layer insulating film 24 of 1 μm or more thick isformed covering the above-structured TFT and inter-layer insulating film22 for planarization of the top surface. A flattening inter-layerinsulating film 24 is made of SOG (Spin on Glass), BPSG(Boro-phospho-Silicate Glass), acrylic resin, or other material. On theflattening inter-layer insulating film 24, a pixel electrode 26 isformed covering the TFT region, for driving the liquid crystal. A pixelelectrode 26 is made of, in the case of a transmission display, atransparent conductive film, such as ITO (indium Tin Oxide), andconnected via the contact hole formed throughout the flatteninginter-layer insulating film 24 to the source electrode 16. For areflection display, conductive reflective material, such as Al, may beused for a pixel electrode 26.

[0036] Also, on substantially the entire part of the TFT substrate 10, avertically-alignment film 28 is formed covering the pixel electrode 26.The vertically-alignment film 28, which is made of a material, such aspolyimide, serves as an alignment film for setting the liquid crystalmolecules to have vertical alignment without a rubbing step.

[0037] The opposing substrate (the second substrate) 30, which issituated opposite to the above TFT substrate 10 having the liquidcrystal layer 40 in-between, is made of glass or other material, similarto the TFT substrate 10. On the surface of the opposing substrate 30facing the TFT substrate 10, RGB color filters 38 are formed in apredetermined alignment so as to correspond to the pixel electrodes 26.On the RGB color filter 38, which is covered by a protection film 36,such as acrylic resin, there is formed a common electrode 32, which ismade of a material such as ITO, for driving the liquid crystal incorporation with the opposing pixel electrode 26. Note that, when an LCDpanel is used as a light valve of a projector, in a system in whichthree panels are used respectively for R, G, B, a color filter 38 isunnecessary because incoming light has already been separated into R, G,B.

[0038] Also, the common electrode 32 has an electrode-free area formedas an direction control window 34 on a part thereof corresponding to thepixel electrode 26 (described later). An direction control window 34 mayhave an X shape. Covering the common electrode 32 and the directioncontrol window 34, a vertically-alignment film 28 is formed, similar tothe TFT substrate 10.

[0039] The liquid crystal layer 40 is a crystal liquid layer enclosed ina space, for example, of 3-5 μm thick between the substrates 10, 30. Theliquid crystal layer 40 is made of liquid crystal material havingnegative dielectric constant anisotropy, which has a larger dielectricconstant in a shorter axial direction than in a longer axial directionof a liquid crystal molecule 42. Liquid crystal material for use in theliquid crystal layer 40 in this embodiment is a mixture of the liquidcrystal molecules having fluorine for a side chain, expressed by thefollowing chemical formulas (1) to (6), at a desired ratio so as tocontain at least one type of these liquid crystal molecules.

[0040] Presently, liquid crystal molecules having cyano(CN—) for a sidechain are mainly used as liquid crystal material with negativedielectric constant anisotropy for use in a TFT LCD including an a-Siactive layer with low mobility. However, liquid crystal molecules with acyano side chain must be driven by a sufficiently high voltage as theywould otherwise be largely affected by a residual DC voltage. Moreover,those LC molecules have a poor voltage holding ratio, and may possiblycause image persistence. On the other hand, this embodiment uses a p-SiTFT which was made using low-temperature processing and is adapted to bedriven by a low driving voltage. If the presently used liquid crystalmaterial having a cyano side chain (which must be driven by a highvoltage), is used in this embodiment, the characteristics of the p-SiTFT, i.e., being adapted to be driven by a low driving voltage, cannotbe utilized effectively. Therefore, the above mentioned liquid crystalmolecules having fluorine for a side change are adequately mixed for usein this embodiment. The resultant liquid crystal layer 40 is adapted tobe driven by a low voltage, and can maintain a sufficiently high holdingrate when driven by a low driving voltage through the p-Si TFT, and evenprevent image persistence. Also, the resultant LCD has lower powerconsumption than an LCD which employs an a-Si TFT, because it can bedriven by a lower voltage.

[0041] Further, since an direction control window 34, which is anelectrode-free region in the common electrode 32, is formed, as shown inFIGS. 1 and 2, liquid crystal molecules are caused to tilt in apredetermined direction, using the direction control window 34 as areference so that response of the molecules is improved. Moreover, sincethe alignment direction is resultantly diverged within a single pixeldue to the window 34, viewing angle dependency of the liquid crystaldisplay is modified, which makes it possible to achieve a display with awider viewing angle.

[0042] Specifically, when a voltage is applied to the liquid crystallayer 40 (a white display, i.e., in a liquid crystal ON state), diagonalelectric fields are generated between the edges of the pixel electrodes26 and the common electrodes 32. The diagonal fields are actuallydiagonal in different directions in respective portions, as indicated bythe broken line in FIG. 2. This causes the vertically-aligned liquidcrystal molecules at the edges of the pixel electrodes 26 to tilt in adirection opposite to the diagonal direction of the electric field.Since liquid crystal molecules 42 have continuity, once the tiltdirection of the liquid crystal molecules at the edge of the pixelelectrode 26 is determined due to the diagonal electric field (the tiltangle is determined according to the electric field intensity), theliquid crystal molecules around the center of the pixel electrode 26 areaccordingly caused to tilt in the similar direction. That is, when apixel is driven, a plurality of regions are caused in a single pixelregion, where liquid crystal molecules tilt in different directions.

[0043] On the other hand, liquid crystal molecules in an areacorresponding to the direction control window 34 remainvertically-aligned, as shown in FIG. 2, since the direction controlwindow 34 always receives a voltage less than a liquid crystal drivingthreshold value. As a result, the direction control window 34 alwaysmakes a boundary between the regions with liquid crystal moleculestilting in different regions. For example, with an X-shaped directioncontrol window 34, as shown in FIG. 1, the boundaries for separating theregions A, B, C, D where liquid crystal molecules tilt in differentdirections are fixed on the X-shaped direction control window 34. Withthis arrangement, direction separation is successfully made within asingle pixel region, and the boundaries for the separation can be fixedon the direction control window 34. Moreover, a plurality of (four inthis embodiment, i.e., upper, lower, right, and left) priority viewingangle directions can be ensured, so that an LCD with a wider viewingangle can be provided.

[0044] Also, the pixel electrode 26 is provided on the inter-layerinsulating films 22, 24, covering the region where the TFT and electrodelines thereof (gate electrode lines, drain electrode lines), and so on,are formed. Therefore leakage of magnetic fields generated by the TFTand its electrode lines into the liquid crystal layer 40 is prevented,as are the resulting effects on the alignment of the liquid crystalmolecules. Further, a flattening inter-layer insulating film 24 canimprove the planarity of the surface of the pixel electrode 26, and canbe prevented the disturbance to the alignment of liquid crystalmolecules due to the uneven surface of the pixel electrode 26. With theabove arrangement with a reduced leaking electric field from the TFT andthe electrode lines and the pixel electrode 26 with a more flattenedsurface, a step of rubbing the vertical alignment film 28 is unnecessaryas the alignment of the liquid crystal molecules is controlled by usinga function of the electric fields caused at the edge portion of thepixel electrode 26 and in the direction control window 34.

[0045] Further, with the above arrangement in which the pixel electrode26 is formed covering the TFT and respective electrode lines, anexcessive alignment margin for the TFT and the lines is unnecessary.This can improve an aperture ratio.

[0046] [Driving circuit]

[0047] Next, a driving circuit and method for improving a response timeof the above-structured DAP LCD panel in a normally-black mode will bedescribed.

[0048] Referring to FIG. 3, which shows a global structure of an LCD ofthis embodiment, the LCD comprises an LCD panel 50 and a driving circuit60 therefor.

[0049] The LCD panel 50 has a display area 52, where a TFT substrate andan opposing substrate sandwich a liquid crystal layer, as shown in FIGS.1 and 2, and low temperature p-Si TFTs are formed as a display TFTs onthe TFT substrate side. Note that the channel, source, and drain of alow temperature p-Si TFT can be formed through self-alignment. Aroundthe display 52 on the TFT substrate, an H driver 54 and V drivers 56 areformed for horizontal and vertical selection of the display TFTS,respectively. These H and V drivers 54, 56 are formed using p-Si TFTseach having a CMOS structure. A p-Si TFT having a CMOS structure isformed in a process substantially same to the process for forming a p-SiTFT for the display area 52 whose channel, source, and drain can beformed through self-alignment. With the above panel structure, a rubbingstep which may adversely affect the closely situated p-Si TFTs for thedrivers 54, 56, can be omitted. This contributes to improving the yieldof an LCD.

[0050] The driving circuit 60 of the liquid crystal panel 50 isconstructed with integrating a video chroma processing circuit 62, atiming controller 64, and other components. The video chroma processingcircuit 62 generates R, G, B video signals by using an input compositevideo signal. Using reference oscillation signals generated by a VCO 66,the timing controller 64 generates various timing control signals basedon an inputted video signal, to supply to the video chroma processingcircuit 62, an RGB driver processing circuit 70, a level shifter 68, orother circuits. Using R, G, B video signals supplied from the circuit62, the RGB driver processing circuit 70 generates AC driving signalsfor R, G, B according to the characteristics of the TFT LCD, to outputto the LCD panel 50.

[0051] In this invention, liquid crystal driving signals are controlledso as to achieve ON display levels set for R, G, B, respectively. Thesetting of ON display levels and the controlling of the levels of liquidcrystal driving signals can be made by using an RGB driver processingcircuit 70 having a structure described later.

[0052]FIG. 4 shows the relationship between impressed voltages [V] andtransmittance [T] for R, G, B.

[0053] As having already been described, transmittance with an ECB LCDlargely depends on a birefringence extent, expressed as Δn·d/λ, andtransmittance of incoming light depends on wavelength. Also, when theliquid crystal materials expressed by the above chemical formations(1)-(6) are used to easily achieve low voltage driving by using a lowtemperature p-Si TFT, the A n value of the materials may be set at, forexample, approx. 0.07. Specifically, it may be set at approx. 0.07 orless for a reflection display. Further, in the case of a reflection LCD,the Δn·d value is desired to be set at 0.3 or less in order to achievelow voltage driving. Therefore, impressed voltage-transmittancecharacteristics of R (Rλ≈630 nm), (Gλ≈550 nm), (Bλ≈460 nm) resultantlydiffer significantly from one another. Also, as shown in FIG. 4, theimpressed voltage for the largest transmittance differs among R, G, B.In the example shown in FIG. 4, the largest transmittance, i.e., approx.475×10⁻³, can be achieved with an impressed voltage of approx. 7 V for Gand approx. 5 V for B. However, it cannot be achieved for R even with animpressed voltage of approx. 8 V.

[0054] In this embodiment, in order to achieve substantially equaltransmittance for R, G, B colors with an ECB having wavelengthdependency, a liquid crystal driving level for the maximumtransmittance, i.e., a voltage level for turning on the liquid crystalpixels, must be set, for example, at approx. 7.8 V for R, 7 V for G, and4.9 V for B, when the respective color components have thecharacteristics shown in FIG. 4. With this setting, the white color canbe precisely displayed for color display in which the white color isdisplayed through composition of R, G, B light.

[0055] Also, in adjustment of driving voltage levels, since thecharacteristics of an impressed voltage-transmittance relationship aredifferent for R, G, B, gamma correction may be given to the liquidcrystal driving signals for R, G, B in, for example, an RGB drivingprocessing circuit 70 according to the respective characteristics. Withthis adjustment, color reproducibility can be enhanced even with respectto intermediate graduation.

[0056]FIG. 5 shows a partial structure of an RGB driver processingcircuit 70 for adjusting an ON display voltage level for every RGBliquid crystal driving signal. Although the structure shown in thedrawing particularly concerns an R liquid crystal driving signal, anidentically structured circuit is provided for each of the other colors,namely G, B. FIG. 6 shows an example of a limit level generation circuit84 shown in FIG. 5; FIG. 7 shows a waveform of a signal which is changedin the RGB driving processing circuit 70 shown in FIG. 5.

[0057] Video signals for R outputted from the video chroma processingcircuit 62 are supplied to a differential output amplifier 73 shown inFIG. 5. In the differential output amplifier 73, the signals aresubjected to brightness adjustment to change the signals so as to have aDC voltage which is determined based on the voltage of a bias circuit72. The differential output amplifier 73 outputs a non-inverted signaland an inverted signal to a first buffer 74 and a second buffer 75,respectively. The first buffer 74 outputs a non-inverted output signala′ having a waveform indicated by the broken line in FIG. 7(a); thesecond buffer 75 outputs an inverted output signal b′ indicated by thebroken line in FIG. 7(b). These signals a′ and b′ are then supplied to afirst limit circuit 78 and subsequently to a second limit circuit 80before being outputted to a multiplexer 82. The first and second limitcircuits 78, 80 defines the lower and upper limit level of the signalsa′ and b′ for every one cycle (see the waveforms indicated by the solidlines in FIGS. 7(a) and (b)).

[0058] The multiplexer 82 alternately selects a non-inverted and aninverted output signal (a), (b) of which levels are controlled with afirst and a second limit circuits, for every predetermined cycle T(e.g., one frame period, one line period, and so on) based on aninversion control signal, to output via a buffer to the LCD panel 50 asan AC driving signal (c) for driving the liquid crystal for R display.

[0059] The first limit circuit 78 comprises a transistor Q1 and atransistor Q2. The transistor Q1 is connected in a signal path betweenthe first buffer 74 and the multiplexer 82; and the transistor Q2 isconnected in a signal path between the second buffer 75 and themultiplexer 82. Transistors Q1 and Q2 receive, at their bases, a firstlevel control signal (d) having a waveform shown in FIG. 7(d) from alimit level generation circuit 84.

[0060] The second limit circuit 80 comprises a transistor Q3 and atransistor Q4. The transistor Q3 is connected in a signal path betweenthe first buffer 74 and the multiplexer 82; and the transistor Q4 isconnected in a signal path between the second buffer 75 and themultiplexer 82. The transistors Q3 and Q4 receive, at their bases, asecond level control signal (e) having a waveform shown in FIG. 7(e)from a limit level generation circuit 84, and operate in response to thereceived second level control signal (e). When the impressedvoltage-transmittance relationship has the characteristics shown in FIG.4, levels of a non-inverted output signal (a) and an inverted outputsignal (b) are controlled in response to the limit levels of thetransistor Q2 of the first limit circuit 78 and the transistor Q3 of thesecond limit circuit 80 so that the ON display levels (an upper limitlevel) of the voltage (absolute value) applied to the liquid crystallayer thereof become at a desired voltage level VRon.

[0061] In response to voltages determined based on the signals (d), (e),the transistor Q1 of the circuit 78 and the transistor Q4 of the circuit80 operate to define the level of a non-inverted signal (a) and aninverted output signal (b), so that the OFF display level of the voltage(absolute value) applied to the liquid crystal layer remains at apredetermined level greater than 0V. As a result, the liquid crystallayer having an initial tilt angle of approx. 0° can be turned on at ahigh speed. Note that although the transistors Q1 and Q4 of the firstand second limit circuit 78, 80 are not indispensable in thisembodiment, they can be provided in the above to control the upper andlower levels of inverted and non-inverted signals so as to remain withina predetermined range for controlling the black display level and forpreventing an excessive voltage from being applied to the multiplexer82, and for enhancing synchronicity between the upper and lower levelsof an AC driving signal (c).

[0062] Next, the structure of limit level generation circuit 84 will bedescribed with reference to FIG. 6. In response to an inversion controlsignal which inverts the level thereof for every cycle (T) of beingsupplied to terminal 100, the limit level generation circuit 84 outputsa first level control signal (d) which varies the level thereof from theemitter of a transistor Q11, and a second level control signal (e) whichsimilarly varies the level thereof from the emitter of a transmitterQ10.

[0063] When the voltage of an inversion control signal supplied toterminal 100 is H level higher than the voltage Vref′ of the referencepower source 86, the transistor Q19 is turned on, and a current I whichis substantially equal to current I₂ supplied from a constant currentsource 92 is caused to flow over resistor R1 by the first current mirrorcircuit CC1. At the same time, the inversion control signal supplied toterminal 200 is L level, so that a reference power source 90-2 (Vref2)is selected and connected to the resistor R₁. As a result, the basepotential of the transistor Q10 is equal to a value “Vref2+R₁·I₂”, andthe transistor Q10 outputs a corresponding second level signal (e) fromits emitter. Also, since the transistor Q14 is then is an OFF state, nocurrent flows in the second current mirror circuit CC2. Therefore, thebase potential of the transistor Q11 remains equal to the voltage“Vref2” of the reference power source 90-2, and the transmitter Q1outputs a first level control signal (d) from its emitter.

[0064] On the other hand, when the voltage of an inversion controlsignal supplied to terminal 100 is a level lower than the voltage Vref′of reference power source 86, transistor Q14, which makes a differentialpair with PNP transistor Q13, is turned on. A current I which issubstantially equal to the current I₁ supplied from current source 88 isthen caused to flow through resistor R2 by a second current mirrorcircuit CC2. As the same time, the inversion control signal supplied toterminal 200 is H level, so that a reference power source 90-1 (Vref1)23 is selected and connected to the resistor R2. As a result, the basepotential of the transistor Q11, which is connected to resistor R2, isequal to a value “Vref1−R₁·I₁”(FIG. 7(d)) due to the voltage drop at theresistor R2, and the transistor Q11 outputs a corresponding first levelsignal (d) from its emitter. Also, since transistor Q19, which makes adifferential pair together with an NPN transistor Q20, is then in an OFFstate, no current flows in the first current mirror circuit CC1.Therefore, the base potential of transistor Q10, which is connectedbetween an output transistor of the first current mirror circuit CC1 andthe resistor R1, remains equal to the voltage “Vref1” of the referencepower source 90-1, which is connected to the other end of the resistorR1. As a result, the transistor Q10 outputs a second level controlsignal (e) shown in FIG. 7(e) from the emitter thereof.

[0065] Waveforms of the first and second level control signals (d) and(e) are shown with double dots chain line in (d) and (e) of FIG. 7.Waveforms shown with solid line in (d) and (e) of FIG. 7 are basevoltage waveforms and coincide limit levels for the non-inverted andinverted signals (a), (b).

[0066] Having received inverted and non-inverted signals from secondlimit circuit 80, the multiplexer 82 alternatively selects the signals.When the inversion control signal is L level and the multiplexer 82selects a non-inverted output signal (a) (a period T1 in FIG. 7), sincethe limit level at transistor Q3 is set at the value “Vref2+R₁·I₂”, theupper level (corresponding to the ON display level at the period T₁) ofa non-inverted output signal (a) is resultantly controlled so as not toexceed “Vref2+R₁·I₂”. Moreover, since the limit level of the transistorQ1 is set at the value “Vref2”, the lower level (corresponding to theOFF display level at the period T₁) of a non-inverted output signal (a)is resultantly controlled so as not to be below “Vref2”. In addition,since transistor Q2 of the first limit circuit 80 controls an invertedoutput signal (b), which is then not selected by multiplexer 82, so asto remain “Vref2”, generation of excessive voltages at a part betweenthe switching terminals of the multiplexer 82 can be prevented.

[0067] On the other hand, when multiplexer 82 selects an inverted outputsignal (b) (a period T2 in FIG. 7), since the limit level at transistorQ2 of the first limit circuit 78 is set at the value “Vref1—R₂·I₁”, thelower level (corresponding to the ON display level at the period T2 ofan inverted output signal (b) is resultantly controlled so as not to belower than “Vref1—R₂·I₁”. Moreover since the limit level at thetransistor Q4 of the second limit circuit 80 is set at the value“Vref1”, the upper level (corresponding to the OFF display level at theperiod T2) of an inverted output signal (b) is controlled so as not toexceed “Vref1”. In addition, since transistor Q3 of the second limitcircuit 80 controls a non-inverted output signal (a), which is then notselected by multiplexer 82, so as to remain “Vref1”, generation ofexcessive voltages at a part between the switching terminals ofmultiplexer 82 can be prevented.

[0068] Since the circuits operate as described above, the signalsupplied to LCD panel 50 from the multiplexer 82 via a buffer iscontrolled, so that the ON display levels at periods T1 and T2 arerespectively controlled to be equal to or less than “Vref2+R₁·I₂”, or tobe equal to or greater than “Vref1−R₂ I₁”.

[0069] In this embodiment, the above processing is carried out on eachof the R, G, B video signals. In this processing, the respective ONdisplay levels VRon, VGon, VBon which are actually supplied the liquidcrystal layer can be set at a desired value by setting the resistancevalues of the resistors R1, R2 and the voltage Vref1, Vref2 of thereference power sources 90-1, 90-2 of the limit level generation circuit84 shown in FIG. 6 at a desired value. Specifically, when an LCD has thecharacteristics shown in FIG. 4, the resistance values of the resistorsR1, R2 and the voltages Vref1, Vref2 of the reference power sources90-1, 90-2 are adjusted for each color so that VRon, VGon, and VBonbecome 7.8V, 7V, and 4.9 V, respectively. Also, when the opticalcharacteristics changing voltage Vth for liquid crystal varies accordingto a temperature change, the voltages Vref1, Vref2 of the referencepower sources 90-1, 90-2 may be changed respectively for R, G, B so thatVon, Voff (an OFF display level) follow the change of Vth. With thisarrangement, preferable color display can be always achieved despite achange in the ambient temperature of an LCD.

[0070] It should be noted that when the ON display voltages for R, G, Bcan be set such that the maximum difference ΔV among the three setvoltages, namely VRon, VGon, and VBon, stays within a relatively smallrange, e.g., within 20%, a driving voltage can be easily adjustedrespectively for R, G, B while minimizing the load imposed on thedriving circuit. For example, when displaying R, G, B colors in one LCDpanel, the driving circuit of the LCD panel often makes RGB liquidcrystal driving signals by using the same power source. Therefore, withΔV of approx. 20%, there is no need to use separate power sources forthe respective colors.

[0071] [Application to Projector]

[0072] In this embodiment, the above structured LCD panel 50 may beapplied as a reflection type LCD to a light valve of a projector. FIG. 8shows a structure with this application. In this case, polarizationfilms 44, 46 shown in FIG. 2 are unnecessary for the LCD panel 50.

[0073] Light from a light source 160 is introduced into a polarizedlight separation filter 162, where polarized light rays in apredetermined direction are separated. The separated light rays are thenintroduced into a first polarization film 164 so that a predeterminedlinearly polarized light only passes through the first polarization film164 to be further introduced into a reflection type LCD panel 50.

[0074] A projector may often be provided with three LCD panels 50 for R,G, B arranged in parallel for each receiving corresponding R, G, B lightray separated from the light of the light source. In this embodiment, ONdisplay voltage levels VRon, VGon, VBon for liquid crystal drivingsignals are determined for every LCD panel 50 according to correspondingR, G, B light. In addition, gamma correction may be applied, ifnecessary, to the liquid crystal driving signals according to therespective characteristics. In the above arrangement, in which separateLCD panels 50 are used for R, G, B, the value Δn·d of each panel may beadjustable in consideration of the characteristics for R, G, B, so thatwavelength dependency of voltage-transmittance characteristics of theliquid crystal layer can be more reliably canceled.

[0075] After being adjusted in a circuit shown in FIG. 5, the liquidcrystal driving signals are supplied to a reflection type LCD panel 50.The LCD panel 50 then controls birefringence of the liquid crystal layerfor every liquid crystal pixel, and the panel 50 reflects the linearlypolarized light from the first polarization plate 164 and thereby thereflected light ejects to the second polarization film 166. The R, G, Blights ejected from the panel 50 and then passed through a secondpolarization film 166 are then composited by a compositing opticalsystem (not shown) into a color image for projection, after beingenlarged, onto a screen 170 by a projector lens 168.

what is claimed is:
 1. A liquid crystal display having liquid crystalsandwiched by a pair of substrates having electrodes for driving theliquid crystal based on a liquid crystal control driving signal for Rlight, a liquid crystal control driving signal for G light, and a liquidcrystal control driving signal for B light to control transmittance of Rlight components, G light components, and B light components for colordisplay, a driving voltage for application to the liquid crystal beingset independently for R display, G display, and B display.
 2. A liquidcrystal display according to claim 1, wherein an upper limit value of arange for the driving voltage is set independently for R light, G light,and B light.
 3. A liquid crystal display according to claim 1, whereinthe liquid crystal control driving signal for R light, the liquidcrystal control driving signal for G light, and the liquid crystalcontrol driving signal for B light are separately subjected gammacorrection based on transmittance characteristics of the R lightcomponents, the G light components, and the b light components.
 4. Aliquid crystal display according to claim 1, wherein the pair ofsubstrates includes a first substrate, electrodes for driving the liquidcrystal formed on the first substrate include a plurality of pixelelectrodes arranged in matrix thereon; and the plurality of pixelelectrodes are connected to corresponding poly-Si thin film transistorseach using a poly-Si layer formed at a low temperature for an activelayer.
 5. An electrically controlled birefringence type liquid crystaldisplay having liquid crystal sandwiched by a pair of substrates havingelectrodes for driving the liquid crystal based on a liquid crystalcontrol driving signal for R light, a liquid crystal control drivingsignal for G light, and a liquid crystal control driving signal for Blight to control transmittance of R light components, G lightcomponents, and B light components for color display, a driving voltagefor application to the liquid crystal being set independently for Rdisplay, G display, and B display.
 6. A liquid crystal display accordingto claim 5, wherein an upper limit value of a range for the drivingvoltage is set independently for R light, G light, and B light.
 7. Aliquid crystal display according to claim 5, wherein the liquid crystalcontrol driving signal for R light, the liquid crystal control drivingsignal for G light, and the liquid crystal control driving signal for Blight are separately subjected gamma correction based on transmittancecharacteristics of the R light components, the G light components, andthe B light components.
 8. A liquid crystal display according to claim5, wherein the pair of substrates includes a first substrate, electrodesfor driving the liquid crystal formed on the first substrate include aplurality of pixel electrodes arranged in matrix thereon; and theplurality of pixel electrodes are connected to corresponding poly-Sithin film transistors each using a poly-Si layer formed at a lowtemperature for an active layer.